The Mechanism of the Pupal Gin Trap

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Abstract
1. Despite partial degeneration of the larval muscles and reorganization of the nervous system, pupae of the privet hawk moth, Sphinx ligustri, show simple behaviour patterns including abdominal flexion and twirling, and a movement which closes the gin trap. 2. The gin trap closes when the motor neurones driving muscles in the ipsilateral half of the adjacent anterior segment discharge together in a brief high-frequency volley. 3. The system which excites the volley is restricted to these motor neurones and is activated only by input at a high frequency from triggering receptors. 4. A second system excites efferent neurones in several segments and is driven by triggering and non-triggering receptors. Therefore at low input frequencies the discrimination between the two kinds of receptors disappears. 5. Motor patterns associated with twirling movements drive the efferent neurones in a sequence which interferes with their simultaneous response to triggering inputs. 6. The arrangement of the closure system shows that the afferent neurones are separated from the efferent neurones by at least one interneurone.